Effect of Redox Reaction Products on the Luminescence Switching

Apr 25, 2013 - The luminescence switching behavior of CePO4:Tb has been widely studied upon an interfacial oxidation–reduction reaction where KMnO4 ...
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Effect of Redox Reaction Products on the Luminescence Switching Behavior in CePO4:Tb Nanorods Guozhu Chen,† Haiguang Zhao,† Federico Rosei,†,‡ and Dongling Ma*,† Centre Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique (INRS), 1650 Boulevard Lionel Boulet, Varennes, Québec J3X 1S2, Canada ‡ Centre for Self-Assembled Chemical Structures, McGill University, Montreal, Québec H3A 2K6, Canada †

ABSTRACT: The luminescence switching behavior of CePO4:Tb has been widely studied upon an interfacial oxidation−reduction reaction where KMnO4 and ascorbic acid act as an oxidant and a reductant, respectively. However, the transformation of Mninvolved species derived from KMnO4 during the oxidation−reduction cycle and their effect on the luminescence properties of CePO4:Tb have not been explored so far. Here, we further study this interfacial reaction between CePO4:Tb and KMnO4 through various characterization techniques, such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. We find that an amorphous manganese oxide layer forms on the CePO4:Tb surface along with the partial oxidation of Ce(III) upon addition of KMnO4. In the subsequent reduction, the ascorbic acid not only reduces Ce(IV) to Ce(III) but also dissolves the formed manganese oxide. If manganese oxide is kept on the CePO4:Tb surface during the reduction treatment, the photoluminescence of Tb(III), due to the energy transfer from Ce(III) to Tb(III), would be restrained even if Ce(IV) ions were efficiently reduced. Although the degree of surface oxidation/ reduction (Ce(III)/Ce(IV)) was considered to be a key factor for the luminescence quenching/recovery behavior in previous studies, there is a strong indication that the reaction product, e.g. manganese oxide, and associated surface defects generated from the oxidation−reduction reaction can disturb the photoluminescence of Tb(III) when they are not removed.

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INTRODUCTION Luminescent nanomaterials are increasingly being used for a wealth of modern technologies, including for example biological applications, such as drug/gene delivery, biosensors, and bioimaging. 1−6 In comparison to conventional organic luminescent materials, lanthanide-doped inorganic systems have been widely studied due to their high photochemical and thermal stability, low toxicity, and long luminescence lifetime.7−14 In addition, some of these lanthanide-doped luminescent materials (e.g., NaYF4:Yb,Er,15 LaPO4:Tb16) are sensitive to local environments and exhibit interesting luminescence switching behavior, rendering them potentially suitable for a variety of applications.17 The interfacial redox reaction, as one of various approaches for the design of luminescent switches, has been widely employed to trigger the luminescence switch by changing oxidation states.18−21 Therefore, studies on interfacial redox reactions can help us understand the luminescent switching mechanism and thus direct their applications correspondingly. Terbium(III)-doped CePO4 (CePO4:Tb) was designed as a luminescence switch based on reversible switching of Ce(III)/ Ce(IV) by the alternative addition of KMnO4 and ascorbic acid.22−25 CePO4:Tb exhibits a strong green emission due to the excitation of Ce(III) followed by an efficient energy transfer (ET) from excited Ce(III) to Tb(III). Under an oxidizing environment (e.g., in the presence of KMnO4), Ce(III) is easily oxidized into Ce(IV), which can efficiently prohibit the ET © 2013 American Chemical Society

from Ce(III) to Tb(III), thus quenching the luminescence of Tb(III) (off state). Subsequent reduction (e.g., by ascorbic acid) of Ce(IV) can induce luminescence recovery, i.e., resume the on-state of CePO4:Tb. Regarding this interfacial reaction, Kitsuda et al.26 studied the relationship between the intensity of the Tb(III) emission and the degree of Ce(III)/Ce(IV) oxidation and demonstrated that the photoluminescence (PL) intensity of CePO4:Tb varies exponentially with the concentration of an oxidant (KMnO4) or a reductant (ascorbic acid). Di et al.27 also established a sensing system for the selective detection of vitamin C on the basis of the redox responsive, reversible luminescence of CePO4:Tb. In addition, CePO4:Tb nanostructures were exploited for biological labeling, where “oxidized” CePO4:Tb did not exhibit any fluorescence within cells, while cells treated with “reduced” CePO4:Tb displayed a measurable level of fluorescence.28 Recently, we successfully synthesized Ce−Mn binary oxide nanotubes by treating Ce(OH)CO3 templates with KMnO4 aqueous solution through an interfacial redox reaction, where the Ce(III) in Ce(OH)CO3 was oxidized to Ce(IV) and MnO4− was simultaneously reduced to manganese oxide, which remains on the nanotube surface.29 Following this work and taking the interfacial redox reaction between CePO4:Tb and Received: March 6, 2013 Revised: April 22, 2013 Published: April 25, 2013 10031

dx.doi.org/10.1021/jp402309f | J. Phys. Chem. C 2013, 117, 10031−10038

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Figure 1. TEM image of the as-prepared CePO4:Tb sample (A) and its HR-TEM image (B). The inset of (B) reports the corresponding FFT pattern from an individual nanorod.

oxidation) aqueous solution followed by stirring for 2 h. The oxidized sample (designated as “O” hereafter) was collected by centrifugation, washed with water and ethanol, and dried at 60 °C in vacuum. During the reduction treatment, the above oxidized sample (“O”) was dispersed in 25 mL of ascorbic acid (10 mM for the largely oxidized CePO4:Tb or 0.4 mM for the slightly oxidized one) or NaBH4 (20 mM). After stirring for 2 h, the reduced sample (designated as “R” hereafter) was collected by centrifugation, washed with water and ethanol, and dried at 60 °C in vacuum. For the oxidation treatment by K2FeO4, 300 mg of asprepared CePO4:Tb sample was dispersed in 25 mL of K2FeO4 (4 mM for high level of oxidation or 0.16 mM for slight oxidation) aqueous solution followed by stirring for 2 h. In the subsequent reduction, the oxidized sample was reduced by ascorbic acid (10 mM for largely oxidized sample or 0.40 mM for the slightly oxidized one). Characterization. The samples were characterized by powder XRD (Bruker D-8 Advance) with Cu Kα radiation, TEM, and high-resolution TEM (HR-TEM) (JEM-2100) equipped with EDS. Micro-Raman spectra were acquired with a RM 1000 Renishaw Raman microscope system equipped with a laser at 758 nm. XPS spectra were acquired using a VG Escalab 220i-XL equipped with a twin anode X-ray source. Fluorescence spectra of samples in solid were taken with a Fluorolog-3 system (HORIBA Jobin Yvon) at room temperature.

KMnO4 into account, we now explore this reaction in greater detail: do manganese oxide species exist on the surface of CePO4:Tb after oxidation with KMnO4? In addition, do such species still exist after subsequent reduction with ascorbic acid, and does their presence influence the PL properties of CePO4:Tb? Although this interfacial reaction has been successfully applied to studying the luminescence switching behavior,22−25 tracking Mn-containing species during the oxidation−reduction cycle is still lacking. Here we fill this gap by investigating the manganese species by means of structural and chemical characterization (e.g., transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray diffraction (XPS), and energy dispersive X-ray spectrometer (EDS)) and further explore its possible influence on the PL properties of CePO4:Tb.

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EXPERIMENTAL SECTION Materials. Cerium(III) nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O), terbium(III) nitrate hexahydrate (Tb(NO3)3·6H2O), potassium permanganate (KMnO4), potassium ferrate (K2FeO4), L-ascorbic acid (C6H8O6), sodium borohydride (NaBH4), and sodium phosphate monobasic monohydrate (NaH2PO4·H2O) were purchased from Sigma-Aldrich and used without further purification. Water was purified by a Millipore Ultrapure water system and has a resistivity of 18.2 MΩ·cm at 25 °C. Synthesis of CePO4:Tb Nanorods and Oxidation− Reduction Treatments. CePO4:Tb nanorods were synthesized by mixing Ce(NO3)3, Tb(NO3)3, and NaH2PO4 aqueous solution. In a typical synthesis process,26 9 mmol of Ce(NO3)3·6H2O and 1 mmol of Tb(NO3)3·6H2O were dissolved in 250 mL of water (atomic ratio: Ce/Tb = 9/1). 250 mL of NaH2PO4·H2O (80 mM) was added into the above mixture. After stirring for 3 h at room temperature, the white precipitate was collected by centrifugation and washed with water and ethanol in sequence. Finally, the as-prepared sample (designated as “A” hereafter) was dried at 60 °C in vacuum. For the oxidation treatment, 300 mg of as-prepared CePO4:Tb sample (“A”) was dispersed in 25 mL of KMnO4 (4 mM for high level of oxidation or 0.16 mM for slight

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RESULTS AND DISCUSSION The as-prepared sample “A” exhibits a rod-like morphology with diameters in the range 6−8 nm and lengths of 80−90 nm, as shown in the TEM micrograph (Figure 1A). The HR-TEM image (Figure 1B) reveals its single-crystalline feature, where the (001) planes are perpendicular to the nanorod’s growth axis. The corresponding fast Fourier transform (FFT) pattern also suggests that the growth of the nanorods occurs preferentially along the [001] direction (c-axis). Following high level of oxidation by KMnO4, and subsequent reduction by ascorbic acid, three samples “A”, “O”, and “R” 10032

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that at most an undetectable amount of Ce(III) is oxidized with KMnO4 treatment. During treatment with KMnO4, the MnO4− should accept electrons from Ce(III) ions to oxidize the latter. An interesting question arises: how is the reduced Mn-based species present, as soluble ions (e.g., Mn2+, Mn4+) or solid (MnOx)? To this end, Mn 2p XPS spectra were also recorded in the oxidized and reduced samples (Figure 3B). Since there is no elemental Mn in initial CePO4:Tb, the Mn 2p XPS spectrum was not taken for sample “A”. Two additional intense peaks appear in the range of 635−670 eV in sample “O”. After indentifying all the elements involved, two peaks can be indexed to match Mn(IV).30 Since the as-prepared sample was extensively washed by water and ethanol, these peaks are not ascribed to the presence of adsorbed Mn4+ ions. Rather, this result confirms the existence of solid Mn-based species on the surface of the CePO4:Tb nanorods. In addition, these two peaks completely disappear in sample “R”, demonstrating that MnO2 was totally removed during the reduction step. Raman spectroscopy was used to identify and measure the change of the manganese-based species in the CePO4:Tb nanorods during oxidation/reduction cycles (Figure 4). As

were characterized with XRD and HR-TEM. As shown in Figure 2, all peaks in samples “O” and “R” can be well indexed

Figure 2. XRD patterns of as-prepared sample “A”, largely oxidized sample “O” by KMnO4, and subsequently reduced sample “R” by ascorbic acid. Insets are the corresponding HR-TEM images of individual nanorods. Black vertical lines represent the hexagonal phase of CePO4 (JCPDS 34-1380).

to the hexagonal CePO4 phase (JCPDS 34-1380) and no characteristic diffraction peaks of Mn-based species can be identified, similarly to that of “A”. Lattice fringes extend continuously along the nanorods in HR-TEM images (insets in Figure 2) except for an indistinct amorphous layer on the surface of the nanorod (indicated with a red circle) in sample “O”. All results indicate that there are no obvious changes in both the crystalline structure and surface morphology of the samples “O” and “R” in comparison to “A”, although an enormous difference exists in their PL spectra, as discussed hereafter. To evaluate the interfacial reaction between CePO4:Tb and KMnO4/ascorbic acid, XPS was used to identify chemical states of surface constituents. Figure 3A presents the Ce 3d XPS spectra of samples “A”, “O”, and “R”. Although KMnO4 is wellknown to be able to oxidize Ce(III) to Ce(IV) and ascorbic acid can reduce Ce(IV) to Ce(III), there is no significant change in the Ce 3d spectra of these three samples, indicating

Figure 4. Raman spectra of the samples “A”, “O”, and “R”.

shown in Figure 4, the Raman spectrum of sample “A” exhibits similar features to that of CePO4, where the strongest band at 978 cm−1 is associated with symmetric stretching vibration of

Figure 3. XPS spectra of Ce 3d (A) and Mn 2p (B) of samples “A”, “O”, and “R”. 10033

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Figure 5. TEM images of largely oxidized samples “O” after oxidation treatment at 25 °C for 6 days (A, B) or at 60 °C for 2 days (C, D); TEM images of the samples reduced with ascorbic acid (E) or NaBH4 (F) from the sample in (C). Inset is the EDS pattern of the sample in (A).

the P−O bond, the 626 cm−1 band can be assigned to asymmetric deformation mode of the PO4 groups, and the bands observed between 300 and 500 cm−1 are associated with the deformation of the PO3 group.31 However, after the oxidation treatment with KMnO4, the intensity of most peaks is suppressed, possibly due to the deep brown color of the “O” sample in comparison to the light color of “A” and “R” samples.32 Since a deep color reduces the effective sample volume probed by the laser beam, the obtained Raman spectra should be considered mainly as indicative of near-surface chemical structures.33 An additional strong band at ∼652 cm−1, characteristic of the Mn−O stretching mode of manganese oxide, was observed. Meanwhile, the band at 578 cm−1, assigned to the deformation mode of the PO3 group, is strengthened due to the formation of manganese oxide after oxidation by KMnO4 (“O” in Figure 4).30,31 These two characteristic peaks fade out and all other peaks return to their original status after reduction by ascorbic acid (“R” in Figure 4). On the basis of XPS and Raman results, we infer that the manganese oxide species forms during the KMnO4 oxidation process while it disappears when sample “O” is reduced by ascorbic acid. To verify the possible mechanism, sample “A” was also largely oxidized by means of extending the reaction time and/or increasing the reaction temperature. When the reaction time was kept for 6 days at 25 °C, an amorphous, thin layer was found on the surface of nanorods (Figure 5A,B). The formation of this amorphous structure was proposed as a result of the incorporation of oxygen ions and Ce3+/Ce4+ change in CePO4 lattice during oxidation.26 However, in our case, elemental Mn was detected in the oxidized sample, as shown in the EDS spectrum (inset in Figure 5). When the reaction temperature was held at 60 °C for 2 days, some irregular, amorphous “flakes” were clearly visible on the nanorod surface (Figure 5C,D). These “flakes” disappeared when the “O” sample was reduced by ascorbic acid (Figure 5E). The Mn signal disappears in these nanorods (EDS, not shown here), consistently with XPS results. In general, the reduction of Ce(IV) into Ce(III) is considered a major factor in determining the recovery of the PL signal from CePO4:Tb. Herein, another strong reducing agent, NaBH4, was also used to reduce sample “O”. In this scenario,

the “flakes” were still observed on the nanorod surface even after NaBH4 reduction (Figure 5F), indicating that, unlike ascorbic acid, NaBH4 can only reduce Ce(IV) but not dissolve the formed manganese oxide. On the basis of the results described above, we propose that manganese oxide forms on the surface due to the reaction between CePO4:Tb and KMnO4. In general, manganese oxide synthesized from interfacial redox reactions, especially under low temperature, has an amorphous structure.29,34−36 Therefore, the amorphous layer in sample “O” is not only due to the incorporation of the oxygen ions into the CePO4 lattice as proposed by Kitsuda26 but also due to the formation of manganese oxide. The most interesting and important phenomenon is that the formed manganese oxide can be dissolved during the reduction treatment by ascorbic acid, but not by NaBH4. It thus offers a unique opportunity to study the influence of manganese oxide on the luminescence switching behavior of CePO4:Tb. The PL properties of all samples were measured in the solid state at least three times, and all the results are consistent and reproducible. For adequate comparison, the concentration of CePO4:Tb was kept constant during reactions for all investigated samples. Representative PL spectra for samples “A”, “O”, and “R” are shown in Figure 6. After the oxidation treatment with KMnO4, the PL peak position does not exhibit any change, while the intensity decreases sharply. This could be attributed to the presence of Ce(IV) on the surface that prevents the ET from Ce(III) to Tb(III).22−25 Subsequent reduction by ascorbic acid almost reproduces the initial emission intensity. However, the emission intensity cannot be recovered efficiently after reduction by NaBH4, even under a higher concentration than that of ascorbic acid. The reduction of Ce(IV) by NaBH4 can be reflected from the color change from dark brown to light (inset in Figure 6). The brown color of sample “O” has been described in terms of the oxidation of Ce(III) in the environment with increased electron density from oxygen and the color changing from brown to white is thus indicative of efficient Ce(IV) reduction.26 Therefore, the loss of the emission intensity in sample “R” treated with NaBH4 mainly results from the disturbance of residual manganese oxide. 10034

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basis of our previous results,29 we infer that iron oxide forms after the interfacial oxidation reaction between Ce(III) and K2FeO4. In the current case, the formed iron oxide cannot be dissolved by ascorbic acid and remains on the surface of the CePO4:Tb nanorods even after reduction. The existence of iron oxide is further reflected from the colors (inset in Figure 7C), which are yellow in “O” and light yellow in “R”, rather than the original white color of “A” and the resumed white color of the sample “R” after the KMnO4 oxidation−ascorbic acid reduction cycle. The existence of foreign species (e.g., manganese oxide or iron oxide) on the CePO4:Tb surface hinders its color recovery after oxidation−reduction treatments. Figure 7C displays the photoluminescence spectra of the samples “A”, “O” treated with K2FeO4, and “R” treated with ascorbic acid. The emission intensity is severely restrained after oxidation with K2FeO4. In addition, the emission of Tb(III) remains low after the addition of ascorbic acid, although in principle the ET from Ce(III) to Tb(III) should be recovered without Ce(IV) interference. The emission intensity is far from the original level of sample “A” even under the treatment of high concentration of ascorbic acid and long reaction time, and it is different from that of “R” treated by the KMnO4−ascorbic acid cycle. This phenomenon also demonstrates that the luminescence switching behavior in CePO4:Tb is not only related to the reversible transformation between Ce(III) and Ce(IV) but also influenced by the foreign species on its surface and associated defects, produced in the oxidation and/or reduction process. In other words, the efficient reduction of Ce(IV) and “clean” surface of the reduced CePO4:Tb are both key factors that determine the recovery of the PL signal. When the CePO4:Tb sample was subject to slighter oxidation by decreasing oxidant concentration (e.g., 0.16 mM vs 4.0 mM KMnO4 or K2FeO4), it was once again found that the emission is switched to the “off’ state after the oxidation treatment. This

Figure 6. Representative photoluminescence spectra of samples “A” (black line), “O” (red line), “R” treated with ascorbic acid (blue line), and “R” treated with NaBH4 (pink line). Insets are the photographs of the samples dispersed in water and corresponding luminescence photographs from solid samples recorded with a digital camera. The excitation wavelength (λex) is 256 nm.

To confirm the effect of “external” surface species on the PL properties of CePO4:Tb, we employed another strong oxidant, K2FeO4, which has been proved to be efficient for oxidizing Ce(III) to Ce(IV)29 and still utilized ascorbic acid to reduce the resultant Ce(IV). As observed in the XPS spectra (Figure 7A), Ce(III) in CePO4:Tb is obviously oxidized in “O” since the peak at ∼917 eV assigned to Ce 3d3/2 for the Ce(IV) state was observed. After the addition of ascorbic acid, as expected, Ce(IV) was reduced to Ce(III). Meanwhile, the Fe signal was detected in both “O” and “R” samples (Figure 7B). On the

Figure 7. XPS spectra of Ce 3d (A) and Fe 2p (B) of samples “A”, “O” treated with K2FeO4, and “R” treated with ascorbic acid. Photoluminescence spectra (C) of corresponding samples (black line for “A”, red line for “O”, and blue line for “R”). The excitation wavelength is 256 nm. Insets in (C) are the photographs of the samples dispersed in water and corresponding luminescence photographs from solid samples recorded with a digital camera. 10035

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Figure 8. Photoluminescence spectra of samples “A”, “O” treated by low concentration (0.16 mM) of KMnO4 (A) or K2FeO4 (B), and “R” treated with ascorbic acid (0.40 mM).

is consistent with the previous result that even a small amount of Ce(IV) is sufficient to significantly quench the luminescence of Tb(III). We also noticed that, no matter which oxidant (KMnO4 or K2FeO4) was employed during these slight oxidation treatments, the emission intensities increase much more for the “R” samples after the addition of ascorbic acid than those of samples subjected to high level of oxidation/ reduction cycle described above. This is reasonable because more manganese oxide/iron oxide and, highly likely, more associated structural defects are introduced onto the surface with high level of oxidation, resulting in a more significant influence on the PL of CePO4:Tb. Under slight oxidation conditions less surface defects are produced, thus imposing minor disturbance on the emission intensity of samples “R”. In contrast, when presynthesized MnO2 powder of equivalent moles of the KMnO4 (that was used to slightly oxidize the CePO4:Tb sample) was physically mixed with the CePO4:Tb nanorods, there is only slight decrease (